The Colorado River has been dammed, diverted, and slowed by reservoirs, strangling the life out of a once-thriving ecosystem. But in the U.S. and Mexico, efforts are underway to revive sections of the river and restore vital riparian habitat for native plants, fish, and wildlife. Last in a series.

The Morelos Dam on the U.S.-Mexico border, where the Colorado’s remaining water is diverted to cities and farms in Mexico. SUPPORT FOR AERIAL PHOTOGRAPHS PROVIDED BY LIGHTHAWK

From the air, the last gasp of the Colorado River is sudden and dramatic. The pale green river flows smack into the Morelos Dam on the U.S.-Mexico border, and virtually all of it is immediately diverted into a large irrigation canal that waters a mosaic of hundreds of fields — alfalfa, asparagus, lettuce, and other vegetables, their vivid green color clashing against the sere desert. The slender thread of water that remains in the Colorado’s channel continues to flow south, but is soon swallowed up by a sea of sand, far short of its delta, which lies 100 miles farther on.

A fisherman on the upper Colorado River in northern Colorado. Low water flows have endangered fish populations and led last year to the closing of parts of the river to fishing.

The Colorado River once surged through the delta during high flows, carrying so much water at times that shallow draft steamboats chugged hundreds of miles up the river into the U.S. with loads of freight. The water in the delta nourished a vast fertile landscape, a fitting end to a river known as the Nile of North America.

“The river was everywhere and nowhere,” the naturalist Aldo Leopold wrote during a 1922 canoe trip to the delta, describing the waterway as it ebbed, flowed, braided, and stalled into pools, nourishing a rich and diverse ecosystem of “a hundred green lagoons,” a “milk and honey wilderness” with thick stands of cottonwoods and willows that provided habitat for hundreds of species of birds. The delta’s marshes, mudflats, and white sand beaches were home to clapper rails, bitterns, mallards, teal, and clouds of egrets.

Bobcats, puma, deer, and wild boar wandered the delta’s forests. Leopold was searching for the jaguar that roamed there, but didn’t see any.

Myriad species of fish swam in the river and in the brackish waters of the Gulf of California, including a relative of the white sea bass, the totoaba, which grew as large as 300 pounds. They were the buffalo of the sea, pursued for their meat and so plentiful that a whole fish sold for a nickel. Fishermen caught 4 million pounds a year in the early 20th century. Also harvesting the bounty of the delta were the indigenous Cocopah, who threaded the waters in dugout canoes and lived in round houses made of reed and brush.

As a natural river, before it was dammed, the Colorado was a massive, dynamic waterway. It flowed from elevations above 14,000 feet in the Colorado Rockies, then dropped to sea level, which meant that it moved at high water with tremendous force, liquid sandpaper carving out red rock canyons. It flooded the desert plains, carving new channels and braids with every inundation. When it receded, it left behind a mosaic of fecund marshes, wetlands, and ponds.

In its natural state, the Colorado had more extreme flows than any river in the U.S., ranging from lows of 2,500 cubic feet per second in the winter to 100,000 cubic feet per second in the summer. In 1884, an all-time historical peak flow reached 384,000 cubic feet per second in Arizona.

But extreme flows are too capricious to support a civilization, so over the past century or so humans have built a network of expensive dams and reservoirs, pipelines, canals, flumes, and aqueducts to tame and divert the flow. Yet these projects also strangled the life out of a once-thriving ecosystem. By design, the river will never again function as a free-flowing stream…